1. Field of the Invention
The present invention relates to a DC arc welding apparatus using high-frequency pulse current, especially higher than 1 KHz.
2. Description of the Prior Art
In general, it has been known that when the Tungsten Inert Gas welding method or the Metal Inert Gas welding method is carried out by using a high-frequency pulse current having higher than 1 KHz as shown in FIG. 1, desirable effects of improvements of an arc stability, a quality of welded product and a welding operation efficiency can be attained in comparison with the case of an application of DC current or pulse current having frequency of less than 1 KHz.
The improved welding effect by the high-frequency pulse current is remarkable depending upon increase of amplitude of the pulse current in the same mean current.
Heretofore, it has been proposed to employ an apparatus shown in FIG. 2 in order to feed the pulse current to the welding part.
However, the conventional apparatus has disadvantages that enough pulse current can not be fed and the power loss in the apparatus is high.
This problem is illustrated with particular reference to FIG. 2 which shows a conventional apparatus.
In FIG. 2, the reference numeral 1 designates a single-phase or three-phase AC power source to supply the power to a main part of the welding apparatus 2 which comprises a main transformer 3, a current limiting reactor 4, a rectifier bridge 5, a switching element 6, a controlling device 7 for the switching element, a rectifier 8, an auxiliary transformer 9, an auxiliary reactor 10 and an auxiliary rectifier bridge 11.
The connections of the power source 1, the main transformer 3, the current limiting reactor 4 and the rectifier bridge 5 and the connections of the auxiliary transformer 9, the auxiliary reactor 10 and the auxiliary bridge 11 are usually connected with two or three cables, though the connection in FIG. 2 is simplified.
A cable 12 connects an output terminal of the main part of the welding apparatus 2 to a welding torch 13 and a workpiece for welding 14 so as to feed the welding current from the main part of the welding apparatus 2 to the arc generating part.
The conventional welding apparatus shown in FIG. 2 operates as follows. When the switching element 6 is turned on and off in the frequency having higher than 1 KHz, under the command of the controlling device 7, the output of the rectifier bridge 5 is short-circuited by turning on the switching element 6, whereby the constant current which is given by the output voltage of the main transformer 3 and the impedance of the current limiting reactor 4 is passed through the switching element 6.
When the switching element 6 is turned off, the constant current is passed through the rectifier 8, the cable 12, the workpiece 14, the welding torch 13, and the cable 12.
As the result, the pulse current is passed between the cable 12 and the arc generating part by turning on and off the switching element 6.
The circuit of the auxiliary transformer 9, the auxiliary reactor 10, the auxiliary rectifier bridge 11 feeds the arc sustaining current having quite small value which is given by the output current of the auxiliary transformer 9 and the impedance of the auxiliary reactor 10, the arc generating part by the output of the auxiliary rectifier bridge 11. Accordingly, when the pulse current in the arc generating part is not fed by turning on the switching element 6 in accordance with said operation, arc between the welding torch 13 and the workpiece 14 is sustained. The rectifier 8 prevents the passage of the output current of the auxiliary rectifier bridge 11 through the switching element 6 when the switching element 6 is turned on.
From the description of the operation, it is understood that the welding current of FIG. 1 is fed through the cable wire 12, the welding torch 13, the workpiece 14 of the conventional apparatus of FIG. 2. In FIG. 2, the switching element 6 is shown as a mechanical contact. In practical circuit, a semiconductor switching element having similar function is used for repeating switch in high-frequency of higher than 1 KHz.
When a transistor is used as the switching element 6, the structure of FIG. 3 can be considered for the switching element 6 and the controlling device 7.
In FIG. 3, the switching element 6 comprises a transistor for switching 15 and the constant-voltage diode 16 connected in parallel to the transistor. The transistor 15 is switched by the signal current fed from the controlling device 7 to the base of the transistor.
The operation of the constant-voltage diode 16 will be stated later. In general, the length of the cable 12 in FIG. 2 can be in a range of several meters to several tens meters in the circumstance of the place for welding operation. In the welding cable, an inductance of about 1.mu. per 1 m is always given. Accordingly, the pulse current having frequency of higher than several KHz is not given as shown in FIG. 1, but is given as shown in FIG. 4.
The disadvantages of the apparatus of FIGS. 2 and 3 are mainly caused by the inductance of the cable 12.
The disadvantages will be discussed referring to FIG. 4. In the conventional apparatus of FIGS. 2 and 3, the output current I.sub.1 of the rectifier bridge 5 is constant regardless the switching operation of the switching element 6 because of extremely high impedance of the current limiting reactor 4. The following equation is given. EQU I.sub.2 + I.sub.3 = I.sub.1 (constant) (1)
wherein I.sub.2 designates a current passed through the switching element 6 and I.sub.3 designates a welding current passed through the arc generating part. When the switching element 6 is turned off at the time t.sub.1 in FIG. 4; that is to stop the signal from the controlling device 7 to the transistor 15, the welding current I.sub.3 does not increase suddenly as shown in FIG. 1 but it increases gradually as shown in FIG. 4 after the time t.sub.1 because of inductance of the cable 12. Accordingly, the current I.sub.2 passing through the switching element 6 cannot be zero immediately after the time t.sub.1 as shown by the equation 1.
However, the transistor 15 of FIG. 3 as a switching element 6 is in the OFF state after the time t.sub.1, whereby the current I.sub.2 is attenuated under passing through the constant-voltage diode 16 and becomes zero at the time t.sub.2 when the current I.sub.3 reaches to I.sub.1.
The welding current I.sub.3 sustains the constant value I.sub.1 until the time t.sub.3. When the switching element 6 is turned on at the time t.sub.3, the current I.sub.3 does not immediately decrease to zero and is attenuated under a time constant given by the inductance and the resistances of the circuit and the arc voltage under passing through the circular circuit of the switching element 6, the diode 8, the cable 12, the workpiece 14, the welding torch 13, the cable 12 and the switching element 6.
However, the resistance in the circular circuit is usually low, whereby the time constant of the attenuation is usually same or longer than the frequency of the pulse current.
Accordingly, the waveform of the welding current I.sub.3 is given by FIG. 4 in the case of relatively short cable 12 and low inductance:
However, if the inductance is high or the frequency of the switching element 6 is high, the next pulse period is started before enough attenuation of the current I.sub.3 so as to turn off the switching element 6. Accordingly, the waveform having small amplitude of the pulse current as shown in FIG. 5 is given and the effect of the pulse current cannot be substantially expected.
The loss caused by the current I.sub.2 passed through the constant-voltage diode 16 of FIG. 3 during the period t.sub.1 to t.sub.2, increases depending upon the increase of the inductance of the cable 12 to remarkably decrease efficiency of the apparatus.